1 When I put on my shoes to take a walk, I know that I'll be walking on the nice solid surface of planet Earth. When I look up and see clouds and blue sky, I know that they make up the atmosphere that surrounds Earth. At night, I can see our one moon in the sky. I can't see the ocean from here, but I know that if I take a drive to the beach, it will be there. This is what makes up a planet -- a good solid surface under my feet, bodies of water, atmosphere up there in the sky, and one moon in orbit.2 At least that is what I thought a planet was, back when the only planet I knew about was Earth. But soon I learned that there were other planets out there.3 People on Earth have known that there were other planets visible in the sky since ancient times. Since Earth was the only planet they had seen up close, they probably imagined that the other ones were similar to Earth too. As astronomers learned more about Mercury, Venus, and Mars, the three planets closest to Earth, they found out that there were differences as well as similarities.